The present inventon relates to a method and apparatus for separating, by means of centrifugal force, coarse and heavy particles from material to be graded. The method in accordance with the invention and the pulp grader, a so called vortex cleaner or cleaner realizing the method, are especially suitable for sorting fiber suspensions in the pulp and paper industry, for example, to separate sand, stones, metallic impurities and like from fiber suspension.
There are many known so called hydrocyclone type cleaners into which pulp to be cleaned is led tangentially into the upper edge of the cylindrical part, whereby the pulp executes a spiral movement circulating downwardly of the wall of the apparatus. At this stage, any heavy impurities, such as stones and metallic particles are drawn under the action of centrifugal force to the outer rim of the turbulent flow against the wall of the apparatus and the lighter material accumulates towards the middle of the apparatus, from where it is discharged most usually via a duct in the upper part of the apparatus having a considerably smaller diameter than the apparatus itself. The heavy material separated from the pump flow is discharged by means of special arrangements via a discharge opening located in the lower part of the apparatus.
In some embodiments, a chamber is arranged protruding downwardly from the lower part of the conical part of a hydrocyclone apparatus and communicating with a duct, which can be closed by a closure device. In the lower part of the chamber there is another closure valve, the opening of which enables the discharge of the chamber. In the normal condition the upper closure device is open and the coarse material being separated flows into the chamber. When the chamber is almost full, the upper closure device is closed and the lower closure device is opened, whereby the material being separated accumulates above the upper closure device. When the chamber is empty the lower closure device is closed and the upper one opened, whereby the material accumulated thereon falls into the chamber.
For example, U.S. Pat. Publication No. 3533506 discloses very specifically problems which are probably encountered when cyclone separators are used. One of the problems, among others, is in the discharge of the cyclone. Particles to be separated by the cyclone can often be of considerable size or otherwise easily clog the discharge opening. When the discharge opening becomes clogged, the rejection or separation of the grader stops and the level of the separated material quickly rises thus filling the cyclone, whereby the particles to be separated are drawn away with the accept pulp. Attempts have been made to prevent or to minimize this clogging of the discharge end of the cyclon, for example, by arrangements in accordance with GB Patent Publication 1249634, which includes both a transverse movable bar extending through a discharge duct into a cyclone cone and an impeller, which both are used in tending to create movement in the separated pulp layer so as to prevent the pulp from clogging the discharge opening of the cyclone, and let the pulp flow uniformly to the discharge duct. The impeller is constructed so as to subject the separated pulp to a force component downwardly towards the discharge opeing, in other words the impeller tends to pump the separated material from the cyclone.
Conventional apparatuses in the present use also have the disadvantage of the filling of the bottom thereof and the stopping of the flow at the bottom of the apparatus, whereby the fractions being separated do not reach the bottom anymore, but are drawn upwards together with the flow of the accept pulp fractions. For the said reason, it has been important with the apparatuses in accordance with the prior art to maintain the level of the accumulated material in the lower part of he apparatus the same or at least below a certain limit so as to prevent the said disadvantageous filling of the bottom.
Additionally, the hydrocyclones, which are used to grade fiber suspensions of the pulp and paper industry, are characterized in that the consistency at which they in accordance with the prior art can treat the pulp, is very low, most usually below 1%, as is stated in the above mentioned US Patent Publication. However, the modern tendency is to reacg continuously higher consistencies in the manufacture processes of pulp and cellulose, which consistencies are in any case clearly above 1%. Thereby the hydrocyclones in their present form make the process more complicated, because all other components connected with the manufacture process can treat thicker pulp, whereby the pulp has to be diluted to the consistency of below 1% before feeding it to the hydrocyclone and has to be thickened it again after the cyclone. This kind of action irrelevant for the actual process involves unnecessary equipment costs and requires treatment, pumping and circulating of considerable amounts of water.
In order to eliminate or minimize said problems and disadvantages, there has been developed a method and apparatus for grading the fiber suspensions in the pulp and paper industry at a consistency which is higher than what is conventionally considered normal. By using the method and apparatus according to the invention it is possible to efficiently separate the heavier impurities from the fiber suspensions upto a consistency of 5%. Thus the apparatus in accordance with the invention is especially suitable, for example, to operate as a presorter for grinders.
The method of grading or removing coarse material from a fiber or pulp suspension in accordance with the invention is characterized in that the rotational speed of the pulp carried to the secondary stage is increased, whereby the accept fraction is separated from the pulp by taking advantage of the centrifugal force and returned back to the pulp circulation, and the reject is discharged from the system.
The apparatus in accordance with the invention is characterized in that a rotatable rotor for accelerating the speed of the rotational movement of the pulp flowing into the reject chamber is arranged in the reject chamber, and that the reject chamber has a discharge duct for the accepted fraction separating from said pulp.